Board Thread:General Discussion/@comment-29678144-20170624151632/@comment-32395702-20170626154210

Someone explained to me that this means your Pokemon were Gen. 6 and under and from before the Port Decca update. That way, when you're trading, someone can know if your rare Pokemon was before the update or after, which would make your rare Pokemon even rarer.

Who knows if that's even true, but, that's what he told me, and it kind of makes sense, tbh.